1. Field of the Invention
The present invention relates generally to the field of electric power generation and distribution systems and, more particularly, to a method and apparatus for effecting controlled restart of electrical service throughout a utility's service area after a power outage.
2. Description of Related Art
In electric power systems, power outages occur for a variety of reasons. Some outages are planned by a utility to temporarily reduce system load during periods of peak loading or to accommodate infrastructure servicing or improvements. However, most outages are unplanned and result from inclement weather, unexpected excessive demand, or system control errors.
In addition, brown-outs may occur when system load has been cut to offset or account for power quality or supply issues. Brown-outs are used to avoid “dropping” or having a catastrophic failure of a grid area for a generating or distributing utility. Each utility's grid area is interconnected with breakers and tie lines between one or more adjacent utilities so that a grid area that becomes problematic can be isolated from other portions of the grid so as not to bring down the entire system.
The restoration of power to utility customers after a blackout or brown-out is generally considered to be a good thing to customers. However, restoring power to many customers simultaneously or within a very short period of time can cause additional problems. For example, power restoration after a planned or unplanned power outage often causes a momentary and sometimes harmful “spike” or surge in power on the utility grid from generation and distribution perspectives. An exemplary graph 100 illustrating power demand versus time during a power restoration period is shown in FIG. 1. As illustrated in the graph 100, the simultaneous restoration of power to many service points in a utility's service area may cause energy demand 101 to surge or spike momentarily to a very high level or peak for the utility. The power spike may temporarily exceed the acceptable peak power supply 103 of the utility, as illustrated in exemplary form in FIG. 1. If the power spike during restoration exceeds the acceptable peak power supply 103 of the utility, the utility may be forced to use some of its reserve power, typically called “operating reserve.” FIG. 2 is a table or chart illustrating a utility's typical energy capacity. As shown in FIG. 2, operating reserve typically includes three types of power: so-called “regulating reserve,” “spinning reserve,” and “non-spinning reserve” or “supplemental reserve.” Spinning reserve and non-spinning reserve are collectively referred to as “contingency reserve.” Therefore, operating reserve generally consists of regulating reserve and contingency reserve. The types of operating reserve are discussed in more detail below as they relate to the present invention.
Atypical changes in demand may occur that are so abrupt that they cause a substantial fluctuation in line frequency within the utility's electric grid. To respond to and correct for such changes in line frequency, utilities typically employ an Automatic Generation Control (AGC) process or subsystem to control the utility's regulating reserve. Thus, the regulating reserve component of a utility's operating reserve is typically limited to correcting for changes in line frequency. In other words, regulating reserve is typically used to regulate line frequency.
On the other hand, normal fluctuations in demand, which do not typically affect line frequency, are responded to or accommodated through certain activities, such as by increasing or decreasing an existing generator's output or by adding new generation capacity. Such accommodation is generally referred to as “economic dispatch.” The contingency reserve component of a utility's operating reserve (i.e., spinning reserve and non-spinning reserve) provides the additional generating capacity that is available for use as economic dispatch to meet changing (increasing) demand.
As shown in FIG. 2, spinning reserve is additional generating capacity that is already online (connected to the power system) and, thus, is immediately available or is available within a short period of time after a determined need (e.g., within ten (10) to fifteen (15) minutes, as defined by the applicable North American Electric Reliability Corporation (NERC) regulation). Non-spinning reserve (also called supplemental reserve) is additional generating capacity that is not online, but is required to respond within the same time period as spinning reserve. Typically, when additional power is needed for use as economic dispatch, a power utility will make use of its spinning reserve before using its non-spinning reserve because the generation methods used to produce spinning reserve tend to be cheaper than the methods used to produce non-spinning reserve or the perceived environmental impact resulting from the production of non-spinning reserve may be less desirable than the perceived environmental impact resulting from the production of spinning reserve. For example, spinning reserve may be produced by increasing the torque of rotors for turbines that are already connected to the utility's power grid or by using fuel cells connected to the utility's power grid; whereas, non-spinning reserve may be produced by turning off resistive and inductive loads, such as heating/cooling systems attached to consumer locations, or by activating a standby (e.g., nuclear or coal-fired) power plant. However, making use of either spinning reserve or non-spinning reserve (i.e., contingency reserve) results in additional costs to the utility due to the costs of fuel, incentives paid to consumers for traditional demand response, maintenance, and so forth. As a result, utilities use their contingency reserve only when absolutely necessary. Use of contingency reserve is typically referred to as “cold load pickup” and may last from seconds to a few minutes.
While the additional costs associated with use of contingency reserve are generally undesired, such costs may outweigh the more onerous result of a restart spike, which may overload and cause damage to some parts of the utility's grid. A damaged grid can cause further power problems to service areas sourced by the grid immediately after a restart. Thus, when restarting electrical service after an outage, utilities are currently required to balance the costs of using contingency reserve against the costs of possible damage to the grid.
Therefore, a need exists for an apparatus and method for effecting a controlled restart within a utility service area that mitigates the likelihood of a restart spike without requiring use of a utility's contingency reserve.